fix: default new sessions to opus[1m] (1M context) instead of opus (200k)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/web/public/session-ui.js

@@ -941,7 +941,7 @@ Object.assign(CodemanApp.prototype, {
       const stored = localStorage.getItem('caseSettings_' + caseName);
       if (stored) return JSON.parse(stored);
     } catch { /* ignore */ }
-    return { agentTeams: false, opusContext1m: false };
+    return { agentTeams: false, opusContext1m: true };
   },
 
   saveCaseSettings(caseName, settings) {